Question for you Audio Buffs...

I would call myself an entry level audiophile and would appreciate any help I can get. 1st thing, question...

What is the best in-wall, wall mounted, or 5inch Depth or less wall channel I can purchase to compliment the system below.

Yamaha RX-4600 7.1 Receiver
Roomed wired for 7.1 surround
Dynaudio C120 Center Channel (need replacement for this)
6 Niles DS6.3 In ceiling Speakers
(surround front, side, back)
1 Dynaudio 30-A Sub

I recently purchased a Plasma screen that I have wall mounted, and my current center channel speaker is too large for the new setup. Of course with Tamber and other details in mind, what would you recommend to compliment the Niles and get the best sound for the buck. I'm looking to spend anywhere near $400-$800 bucks for the center.

Your expertise is much appreciated!

You should keep everything "voiced" the same to have a great sound field. If it were me, I would use either 1 or 2 matching Niles for the center.
In my opinion, a $400 niles will sound better than many $1000+ speakers that don't match.
I like Niles also. But how are you using ceiling speakers for the fronts? Why not use in-walls for the front channels as well as the center?
Good point.

When the speakers were wired for the room, the installer placed 2 in-ceiling surround fronts (almost in the front corners of the room) - 2 in-ceiling surround left & rights at the sides of the sitting area, and 2 in-ceiling rears behind the sitting area.

He said that they were directional speakers and aimed the tweeters toward the sitting area.

The current center channel speaker acts (and sounds) like the strongest directional speaker in the setup. It's a pretty solid sound field especially in 7-Stereo channel. I agree now that it may have been better to install the 2 surround fronts in the wall instead of the ceiling.

There are a variety of Niles options for the front center. Do I have to get the exact same speakers, or can I mix and find a center from the Niles family?

Niles does make a center that is nice but it can be used for left, right and rear also.